These steps describe my own journey... you will need to adapt them for your own unique set of circumstances. Standard disclaimer applies, as always: You are 100% responsible for your own actions. Using this guide, visiting a link, downloading a program, in short, living, is done entirely at your own risk (and joy).

I used dd_rhelp (as recommended by the author of Knoppix Hacks) and it did a wonderful job. However, in dd_rhelp's readme, the author states that GNU ddrescue is superior to his own tool, so you may want to try that instead:

While testing, I managed to use up all 8 loop devices, and began receiving a "could not find any free loop device" error. Rebooting would have solved the problem of course, but a little Googling turned up the losetup command. List all loop devices with losetup -a and delete loops with losetup -d /dev/loop#.

In my experience, File Scavenger has mounted NTFS disk images that Linux/NTFS-3G could not (despite trying ntfsfix, forcing, etc). I have also seen it recover virtually all desired data from a hard drive that was: partitioned, formatted, subjected to a Windows Vista install, and bombarded with new apps.

DataRescue's DD (DrDD) can image drives reading either forwards (the default) or backwards (which disables the look ahead buffer, thus reducing the risk of lock up on a full track). Windows and Mac binaries available.

ddrescue can be interrupted with Ctrl+C, and will resume from where it left off. You can also resume using the -C option:sudo ddrescue -C /dev/disk2s5 ~/recovered.dmg some_log_name.log

Step III.7 was necessary for me, as attempting to mount the dmg file directly returned an error ("no mountable file systems").UPDATE: The following command might have been able to mount the image by bypassing the file system and disk structure check:hdiutil attach -noverify -noautofsck recovered.dmg

Without the force option in step III.11, umount would always return "Resource busy".

A volume that refused to mount was imaged with ddrescue, but the image would not mount either. Running hdiutil mount -nomount -readwrite foobar.dmg allowed the image to appear in DiskWarrior, which was able to repair the image and make it mountable. See OS X: Imaging and repairing a volume that won't mount for details.